1. Field of the Invention
The present invention generally relates to wireless devices comprising a cluster of antennas coupled to a signal processing device and a method of constructing such devices.
2. Description of the Related Art
One of the more critical pieces of equipment in a communication network and, in particular, in a wireless communication network is the antenna. Antennas are used to convey information (i.e., transmit and receive information) in the form of electromagnetic waves over communication links of a network.
The owners and/or operators of communication networks, i.e., the service providers, are constantly searching for methods and equipment that can meet the changing needs of their subscribers. Subscribers of communication networks, including wireless communication networks, require higher information throughput in order to exploit the expanding range of services being provided by current communication networks. For example, wireless communication subscribers are now able to have simultaneous access to data networks such as the Internet and to telephony networks such as the Public Switched Telephone Network (PSTN). Also, service providers are constantly investigating new techniques that would allow them to increase their information transfer rate. Information transfer rate is the amount of informationxe2x80x94usually measured in bits per secondxe2x80x94successfully conveyed over a communication channel. The information transfer rate can be increased in a number of well known manners. One way is by increasing the power of the transmitted signals. A second way is by expanding the frequency range (i.e., bandwidth) over which the communication is established. However, both power and bandwidth are limited by certain entities such as governmental and standards organizations that regulate such factors. In addition, for portable devices, power is limited by battery life.
An approach that circumvents the power and bandwidth limitations is to increase the number of antennas used to transmit and receive communication signals. Typically, the antennas are arranged as an array of antennas. Three of the more general ways of using antenna arrays are (a) phased array applications, (b) spatial diversity techniques (c) space-time transmit diversity techniques as well as (d) more general Multiple Input Multiple Output (MIMO) techniques. A phased array comprises an antenna array coupled to a device, which controls the relative phase of the signal in each antenna in order to form a focused beam in a particular direction in space. Spatial diversity is the selection of a particular antenna or a group of antennas from an array of antennas so as to transmit or receive signals in order to improve information throughput. In a spatially diverse structure the antenna array is typically coupled to a receive diversity device that utilizes one of many combining techniques, such as Maximum Ratio Combining, switching, or other combining techniques well known to those skilled in the art. Unlike phased arrays and spatial diversity techniques wherein one or a group of antennas are used to transmit or receive a single signal, space-time transmit diversity and MIMO techniques use an antenna array coupled to a signal processing device to simultaneously transmit and/or receive multiple distinct signals. Space-time transmit diversity coding (STTD) uses two or more transmitting antennas in order to take advantage of both the spatial and temporal diversity of the channel; WCDMA for UMTS, p. 97, ed., H. Holma and A. Toskala.
One of the main features of MIMO systems is that they benefit from the multipath propagation of radio signals. In a multipath environment, radio waves transmitted by an antenna do not propagate in straight lines towards the receive antenna. Rather, the radio waves scatter off a multitude of objects that block the direct path of propagation. Thus, the environment creates a multitude of possible paths from transmit to receive antennas. These multiple paths interfere with each other at the location of the receive antenna. This interference process creates a pattern of maxima and minima of received power, with the typical spatial separation between consecutive maxima being approximately one wavelength. MIMO systems exploit the rich scattering environment, and use multiple transmitters and receivers to create, in effect, a plurality of parallel subchannels each of which carries independent information. For transmitting antennas, the transmitted signals occupy the same bandwidth simultaneously and thus spectral efficiency is roughly proportional to the number of subchannels. For receiving antennas, MIMO systems use a combination of linear and nonlinear detection techniques to disentangle the mutually interfering signals. Theoretically, the richer the scattering, the more subchannels that can be supported.
While MIMO techniques theoretically allow antenna arrays to have relatively high information rates, the actual achieved information transfer rate will greatly depend on how the information is coded in the different subchannels. An example of how a MIMO system can be implemented is the BLAST (Bell Labs LAyered Space Time) scheme conceived by Lucent Technologics headquartered in Murray Hill, N.J. There are several realizations of the general BLAST architecture. One of them is known as diagonal-BLAST, or D-BLAST, proposed by G. J. Foschini and M. Gans, Wireless Commun. 6, 311 (1998). Another alternative includes vertical-BLAST, or V-BLAST (proposed by G. D. Golden, G. J. Foschini, R. A. Valenzuela, and P. W. Wolniansky, Electronic Letters 35, 14 (1999)). These implementations can reach a significant (above 80%) fraction of the theoretical information transfer rate expected for rich scattering environments.
As with the idealized MIMO case, in all BLAST implementations the information transfer rate of the system increases as the number of antennas in a transmit and/or receive array is increased. However, in many cases the amount of space available for the antenna array is limited. In particular, the space limitation is very critical for portable wireless devices (e.g., cell phones, Personal Digital Assistants (PDA)). Increasing the number of antennas in an array of limited space decreases the spacing between individual antennas in the array. The reduced spacing between antennas typically causes signal correlation to occur between signals received from different antennas. Signal correlation reduces the gain in information transfer rate obtained by the use of MIMO techniques; A. L. Moustakas et al., Science 287, 287 (2000).
Correlation is quantitatively defined in terms of at least two signals. When any two signals s1(t) and s2(t) are being transmitted or received, the degree of correlation between these two signals is given by the absolute value of the following expression:             ∫              t        1                    t        2              ⁢                            s          1                ⁡                  (          t          )                    ⁢                                    s            2                    ⁡                      (            t            )                          *            ⁢              xe2x80x83            ⁢              ⅆ        t                                ∫                  t          1                          t          2                    ⁢                                    "LeftBracketingBar"                                          s                1                            ⁡                              (                t                )                                      "RightBracketingBar"                    2                ⁢                  xe2x80x83                ⁢                  ⅆ          t                ⁢                              ∫                          t              1                                      t              2                                ⁢                                                    "LeftBracketingBar"                                                      s                    2                                    ⁡                                      (                    t                    )                                                  "RightBracketingBar"                            2                        ⁢                          xe2x80x83                        ⁢                          ⅆ              t                                          
where s2*(t) corresponds to the complex conjugate of s2(t) and t1 and t2 are times selected in accordance to rules well known to those skilled in the pertinent art. When two signals have a relatively low correlation or are uncorrelated, the above integral becomes relatively small.
In particular, received signal correlation is a phenomenon whereby the variations in the parameters (i.e., amplitude and phase) of a first signal of a first antenna track the variations in the parameters of a second signal of a second antenna in the vicinity of the first antenna; Microwave Mobile Communications, W. J. Jakes (ed.), chapter 1, IEEE Press, New York (1974). Also, the correlation between received signals can be determined by the correlation of the radiation patterns of the antennas receiving the signals. As is known to those skilled in the art, the radiation pattern of a particular antenna is the relative amplitude, direction and phase of the electromagnetic field in the far field region radiated at each direction. The radiation patterns are reciprocal in that they show the relative amplitude, phase and direction of a field transmitted from an antenna as well as the sensitivity of that antenna to incoming radiation from the same direction. The radiation pattern can be measured experimentally in an anechoic chamber, or calculated numerically with the use of a programmed computer.
Typically, the radiation pattern originates from a port of an antenna. A port is a part of the antenna at which a signal is applied to produce electromagnetic radiation or a point on the antenna from which a signal is obtained as the result of electromagnetic radiation impinging on the antenna. In general, an antenna may have more than one port. Cables which are typically used to connect the ports to a signal processing device are not considered part of the antenna. The radiation pattern of a port of an antenna is the antenna radiation pattern resulting after exciting only that particular port. The radiation pattern of a port of an antenna generally depends on many factors. The factors affecting the radiation pattern of a port of an antenna include the placement of the port, the materials from which the port and antenna are constructed, the structure and shape of the antenna, the relative position of the antenna in an antenna array, the relative position of the antenna within a communications device, as well as the position of other objects proximately spaced to the antenna. The reason for the radiation pattern""s dependence on the aforementioned factors is electromagnetic coupling of the antenna to nearby objects. In general, electromagnetic coupling of an antenna to other objects or other antennas can modify the radiation pattern of one or more of the ports of the antenna.
The radiation pattern at a particular frequency of an antenna port in a particular array has several well-known characteristics. One such characteristic is a node or a null. A node or a null is a direction in space where the transmitted (or received) radiation power is zero or relatively small, e.g., more than 20 dB below the average radiated power. Another property is a lobe, which is a direction in space where the radiated power has a xe2x80x98local maximumxe2x80x99. A direction in space where the radiated power is at its highest measured value (commonly referred to as xe2x80x98absolute maximumxe2x80x99) is called the main lobe of the port. A lobe generally has a width, corresponding to the directions around it that have appreciable radiated power. The width of the lobe is defined as the set of directions in the immediate neighborhood of the local maximum which has a radiated power of more than half the value of the local maximum. Also, two lobes from two different radiation patterns at the same frequency are considered as not overlapping if their respective widths do not overlap.
It is useful to describe the radiation pattern in terms of the radiation pattern of an ideal dipole antenna since many antennas have patterns that are similar to those of dipole antennas. A dipole radiation pattern is defined to have a null in two opposite collinear directions and a peak radiated power in the plane perpendicular to the collinear direction, with the power in that plane fluctuating by no more than 5 dB. Such a radiation pattern is said to be polarized along the axis of the nulls. When two ports of a given antenna have dipole radiation patterns that have null axes with relative angles higher than 20 degrees, the antenna is dually polarized at a given frequency when only these 2 ports are operating at that frequency. If the dually polarized antenna has axes with relative angles between 70 and 110 degrees, it is said to be cross-polarized. Similarly, if m ports of an antenna, with m equal to 3 or greater, have dipole radiation patterns, such that any two axes have a relative angle greater than 20 degrees, then the antenna is m-fold polarized at a given frequency when all m ports are operating at that frequency.
The correlation function of two radiation patterns is a useful measure of the degree of their overlap. It is defined as the magnitude of       ∫          xe2x80x83        ⁢                  ⅆ        k            ⁢                                                  E              1                        →                    ⁡                      (            k            )                          ·                                                            E                2                            →                        ⁡                          (              k              )                                *                                ∫                        ⅆ          k                ⁢                              "LeftBracketingBar"                                                            E                  1                                →                            ⁡                              (                k                )                                      "RightBracketingBar"                    2                ⁢                  ∫                                    ⅆ              k                        ⁢                                          "LeftBracketingBar"                                                                            E                      2                                        →                                    ⁡                                      (                    k                    )                                                  "RightBracketingBar"                            2                                          
E1(k) and E2(k) are the far field vector electric fields at direction k of the radiated field at a given frequency due to ports 1 and 2 respectively and E2(k)* is the complex conjugate of the far field vector electric field at direction k due to port 2. The correlation between radiation patterns can be calculated based on the experimentally determined or numerically calculated individual radiation patterns.
When two antennas are placed sufficiently far from each other, the correlation of their radiation patterns at the same frequency will be very small. A result of this effect is that the received signal from two antennas spaced sufficiently apart in a rich scattering environment will be uncorrelated. Typically, it is recommended that to avoid strong correlation the distance between the antennas should be at least       λ    2    ,
where xcex is equal to c/f which is the wavelength corresponding to the largest frequency f within a band of frequencies being used for communication by the antennas, and c is a well-known physical constant representing the speed of light in vacuum; Microwave Mobile Communications, W. J. Jakes (ed.), chapter 1, IEEE Press, New York (1974). Low correlation among the radiation patterns of the different antennas in the array is an essential condition to ensure the good performance of the array when used for a MIMO system. However, many wireless devices, particularly portable wireless devices, provide relatively little space for an antenna array.
One approach that has been proposed for packaging many antennas into a small space is to construct an array of individual antennas; Vaughan et al., U.S. Pat. No. 5,771,022; xe2x80x9cClosely Spaced Monopoles for Mobile Communicationsxe2x80x9d, Rodney G. Vaughan and Neil L. Scott, Radio Science vol. 28, Number 6, PP 1259-1266 (1993). In this antenna array approach, several individual antennas with various desirable engineering properties (e.g., high gain, lightweight, small, easily manufacturable), are assembled into an antenna array. It is found that under certain circumstances individual antennas can be spaced a small fraction of xcex (less than 0.2xcex, for example) and even with the electromagnetic coupling between the antennas, the correlation between signals received at the two antennas can remain smaller than 0.7. Further, the array is to be coupled to a combining stage to process a single communication channel. In addition this approach uses the antenna only for receiving signals; it does not address the issue of simultaneous transmission and reception of multiple distinct signals as required by MIMO applications. Further, this approach does not address the specific space constraints imposed on the size of the array by portable wireless devices such as cell phones and PDAs. The antennas in the array are dipole wire antennas which usually operate well for an antenna length of xcex/2 and therefore cannot meet the space constraints of many portable devices.
Thus, in order for many portable wireless devices performing MIMO operations to achieve relatively high information transfer rate, they need to use an antenna array that allows the simultaneous transmission and reception of uncorrelated signals. Such an array can be produced by separating the antennas in the array by at least half a wavelength. However, an antenna separation of at least half a wavelength would result in arrays too large and cumbersome for relatively small devices (e.g., PDA""s, cell phones). What is therefore needed is a MIMO system comprising a multiple signal processing device coupled to a compact antenna array capable of transmitting and/or receiving uncorrelated signals.
The present invention is a wireless communication device and a method for configuring an antenna cluster used in such a device. The wireless communication device of the present invention comprises a cluster of multiple port antennas coupled to at least one signal processing device where the cluster occupies a relatively small volume of space and the wireless communication device is able to simultaneously transmit and/or receive multiple uncorrelated communication signals.
In the antenna cluster each antenna port operates within a frequency band having maximum frequency f. The antennas within the cluster are arranged such that at least one pair of antenna ports is placed within a volume whose longest linear dimension is xcex/3 or less where xcex is equal to c/f. The cluster comprises N antennas where N is an integer equal to 2 or greater. Each operating antenna port has a radiation pattern representing the relative amplitude levels and phase values of the electromagnetic waves being received and or transmitted by the antenna port along different directions. The coupling between antenna ports causes their respective radiation patterns to be modified. In a preferred embodiment, each of the antennas in the cluster contains dielectric material; such antennas are commonly referred to as dielectric antennas. The dielectric materials promote the modification of the radiation patterns, as well as allowing for the construction of smaller antennas without reducing their efficiency.
The positioning and orientation of the antennas and thus the construction of the antenna cluster is done in accordance with the method of the present invention. The positioning of the antennas with respect to each other and with respect to the signal processing device is such that their corresponding radiation patterns have main lobes that face different directions and radiation patterns with correlation of less than 0.7 between them. The positioning and orientation of the antennas in the cluster is an iterative process whereby the resulting correlation between radiation patterns is measured and the direction of the main lobe of the pattern is determined. The antennas are thus positioned to achieve relatively high information transfer rates.